Over the last 15 years, scholars have collected and analyzed data on the 125 or so civil wars that have taken place around the world since 1940. Two findings suggest that the outlook for Iraq is significantly more pessimistic than policymakers in the U.S. or Iraq would hope.

The first is what academics Paul Collier and Nicholas Sambanis call the conflict trap. A country that has experienced one civil war is much more likely to experience a second and third civil war…

The second finding is what I call the settlement dilemma. Combatants who end their civil war in a compromise settlement — such as the agreement to share power in Iraq — almost always return to war unless a third party is there to help them enforce the terms. That’s because agreements leave combatants, especially weaker combatants, vulnerable to exploitation once they disarm, demobilize and prepare for peace. In the absence of third-party enforcement, the weaker side is better off trying to fight for full control of the state now, rather than accepting an agreement that would leave it open to abuse in the future.

Iraq today faces both of these problems…

Right now, U.S. forces serve two important purposes. First, they signal to Maliki and the dominant Shiite population that a decisive victory over the Sunnis and Kurds will not be possible. They also signal to the less-numerous Sunni and Kurdish populations that both of these groups will be protected from Shiite exploitation over time. Remove U.S. forces and U.S. involvement in Iraq and you simultaneously embolden the Shiites while telling the weaker groups they must fend for themselves.